WATER APRIL 2013
140
Technical Features
INTRODUCTION
There is an increasing need for water utilities
to provide tangible evidence that targeted
improvement measures in multi-use drinking
water supply catchments can lead to
reduced water quality risks and save cost, as
compared to other improvement measures
(such as water treatment upgrades).
As a rst step to canvass the potential
success of introducing an accelerated stock
exclusion program in a drinking water
supply catchment, the Kersbrook Water
Quality Improvement Project -- Pathogen
Reduction, considered the feasibility of
implementing an accelerated pathogen
reduction effort (primarily stock exclusion
from watercourses) from two perspectives:
the effectiveness of stock exclusion on
water quality (in this case pathogen
concentrations); and the ability to engage
target landholders and realise an effect
in a short timeframe.
The aim of the initial feasibility study,
presented here was to design a three-
year program that could act as a 'proof
of concept' for the effectiveness of such
catchment measures.
CONTEXT
The speci c objectives of the feasibility
study were to:
1. Con rm primary pathogen sources in
the study catchment, Kersbrook Creek
(Adelaide's drinking water supply
catchment), and identify whether
fast-tracking of measures (e.g. stock
exclusion) have the potential to achieve
the desired pathogen reduction targets
(based on community pro ling and
practical implementation factors);
2. Determine spatially targeted mitigation
measures that deliver the desired
pathogen reduction target in the
most cost-effective manner; and
3. Provide a conceptual monitoring and
an evaluation plan that allows the
project to measure the effectiveness
of the mitigation measures.
Background information on the
Kersbrook Creek catchment, results from
the landholder surveys and pathogen water
quality data have been collated and are
summarised in Table 1.
The Kersbrook Creek catchment
was divided into 10 sub-catchments or
'watersheds' based on an interpretation
of a one-second (30m) elevation model of
the area. Landholder surveys and pathogen
modelling were undertaken considering
these spatial boundaries.
The landholder survey illustrated that
watershed areas numbered 3 and 5
within Kersbrook Creek sub-catchment
have a higher level of landholders
willing to participate in the project.
These two watersheds were, therefore,
suggested to become a focus for stock
exclusion/watercourse fencing under the
implementation stage of the project, with
a focus on achieving high rates of uptake
among those landholders. Monitoring for
mitigation effectiveness would focus on at
least one of these watersheds, with other
watersheds used as control sites to provide
statistically robust assessments.
The second spatial scale would include
the Kersbrook Creek sub-catchment as a
whole and also consider the implementation
requirements if works were to be rolled out
across other catchments. As a result, the
pathogen modelling has been completed
for the entire sub-catchment (with results
presented for each watershed). Future
consideration of social/behavioural aspects
and the potential application of market-
based or other policy instruments should
focus on the broader catchment.
PATHOGEN MODELLING
A catchment pathogen model was created
using the 'materials budgeting' approach
(analogous to the approach used for
sediment and nutrient budgeting) to
provide a framework familiar to catchment
management professionals (Ferguson,
2005; Ferguson et al., 2003 in Olley and
Deere, 2003).
The 'pathogen budget' was expressed
in terms of two outputs for each watershed:
• Daily primary pathogen stock The
quantity of pathogens generated each
day in the sub-catchment, considered to
represent the standing stock. Traditional
materials budgets, such as those for
sediments, represent stocks as including
the build-up of stocks over time.
However, pathogens are not conservative
(they degrade over time), so the daily
stock is a more appropriate indicator
in setting priorities for management;
• Daily peak event ux deposited within
and mobilised by catchment run-off
The quantity of pathogens contributed
through direct deposition and transferred
to watercourses via catchment run-off
from source areas each day during
indicative peak events. 'Peak events'
were not precisely de ned but were
considered to be of the order 20mm
falling on a pre-wetted catchment
over around three hours.
The rst step in developing a pathogen
budget is to identify the processes
that govern pathogen sources and their
subsequent fate and transport within
catchments, and to develop these into
a conceptual model. The conceptual
model is then used as a framework for the
development of a mathematical model. The
conceptual model developed for Kersbrook
Creek sub-catchment is given in Figure 1.
The assumptions and values used for the
Kersbrook Creek sub-catchment baseline
model are summarised in Table 2, which also
includes a qualitative indicator of the level
of certainty relating to those assumptions.
The baseline pathogen budget coef cients
used within the model were derived from
literature reviews, and current work by the
model developers and their colleagues
(Table 3). These parameters, along with
speci c criteria of stock grazing patterns,
were included in the model.
Land use (ha) -- is the total area of land
within this class as determined from the
land use GIS layer.
Grazing (ha) -- is the estimated proportion
of the total area (as speci ed in 'Land use'),
which would be used for grazing animals
PROOF OF CONCEPT APPROACH
How can the effectiveness of stock exclusion
on catchment water quality be assessed?
K Billington, J Frizenschaf, D Deere, M Krogh
CATCHMENT MANAGEMENT